1. Field of the Invention
The present invention relates to information recording/reproducing devices, and in particular, to a system for detecting a position of a write/read head for use in information recording/reproducing devices such as optical disc systems.
2. Description of the Prior Art
An information recording disk, for example, an optical disk has a plurality of information recording tracks onto which information is read using an appropriate transducer or write/read head. The head comprises a light source such as a semiconductor laser for emitting a light beam and an optical system for focusing the light beam as a small beam spot onto a track so as to write or read information onto or from the track. In order to write or read information onto or from a desired one of the tracks, the head must be correctly positioned at the desired track.
In the prior art, a desired head travelling amount is set as a position difference between a current position of the head and a desired track position. An actual travelling amount is subtracted from the desired head travelling amount to produce a position error signal becomes zero so that the head is positioned at the desired track.
U.S. Pat. No. 4,615,023 by Inada et al., assigned to NEC Corporation, discloses a head position detector mounted on the head for detecting the current position of the head so as to detect the actual head travelling amount. However, measurement of the actual head travelling amount by the head position detector is not based on detection of the number of tracks crossed by the head so that high measuring accuracy is not expected.
In the prior art, a known optical disk system is further provided with a tracking error detecting device for detecting deflection of the small beam spot from the desired track as a tracking error signal. The optical system, in particular an objective lens in the optical system, is controlled until the tracking error signal becomes zero to position the small beam spot at a center of the track. The above-described U.S. Patent also discloses such a tracking error detecting device as a tracing position detector.
When the head moves so that the small beam spot crosses those tracks, the tracking error signal is an alternating (AC) signal having an alternating rate corresponding to the track crossing rate. Accordingly, it may theoretically be possible to count a track crossing time number by detecting zero-cross points of the tracking error signal.
However, the tracking error signal includes high frequency noise components caused by track position information previously recorded on the disk etc. Accordingly, it is difficult to reliably detect the zero cross points of the tracking error signal.